A soybean contains a large amount of high-quality proteins and has been utilized as an excellent protein source from old times. In particular, soy protein isolate is useful as a food material by virtue of its high protein content and various functional characteristics such as emulsification properties, gelation properties, water holding properties etc.
The soybean protein is composed of various proteins of complicated higher-order structure which are classified into 2S, 7S, 11S and 15S proteins etc. based on e.g. ultracentrifuge sedimentation rates, and these proteins have different characteristics even in physical properties.
For example, the soy protein isolate obtained by acid-precipitating soy milk extracted from de-fatted soy flakes with water consists essentially of 7S globulin (mainly .beta.-conglycinin) and 11S globulin (mainly glycinin), and each component has inherent functional characteristics. However, these components are present in the form of their mixture and thus the inherent functional characteristics of each component cannot sufficiently be utilized in practical use.
Therefore, many attempts have been made to fractionate each component in order to utilize its inherent functions. For example, there are studies and reports of Wolf et al. and Thanh et al. on experimental fractionation, and proposals have been made in Japanese Patent LOP Publication Nos. 56843/1973, 31843/1974, 86149/1976, 124457/1980, 153562/1980, 64755/1981, 132844/1982 and 36345/1983. However, these prior methods are still in the experimental stage and are not suitable for industrial fractionation.
Under these circumstances, it is proposed in Japanese Patent LOP Publication 187755/1986 that soybean protein components can be fractionated in an industrial separation method using pH and temperature regulation in the presence of sulfite etc., but troublesome pH and temperature controls are essential in this method.
There are also many investigations for functional improvements by use of proteolytic hydrolysis with proteases. For example, Japanese Patent Publication No. 24262/1973, Japanese Patent Publication No. 1028/1980, Japanese Patent LOP Publication No. 232341/1987, Japanese Patent Publication No. 14941/1992 etc. are concerned with such improvements, but all these methods are related to functional modifications such as in solubility, non-gelation properties etc. by preceding thermal denaturation of soybean protein for promotion of hydrolysis prior to enzymatic reaction, and there are no attempts at functional modifications such as decomposition of only a specific component in soybean protein.
It is often hard for an native form of protein including soybean protein to undergo decomposition with a hydrolytic enzyme such as protease (S. S. Nielsen et al., J. Agric. Food. Chem., 36, 869 (1988)), and thus protein denaturation by heating, alcohol etc. is common practice prior to proteolytic hydrolysis.
The soy protein isolate is a mixture consisting essentially of 7S globulin (mainly .beta.-conglycinin) and 11S globulin (mainly glycinin) as stated above, and it is known that there is a difference between the components in degree of denaturation caused under the same conditions. For example, it is known that 11S globulin is denatured easier at acidic pH than 7S globulin (I. Koshiyama, J. Sci. Fd Agric., 23, 853 (1972)), and also that 7S globulin is denatured by heating at a lower temperature than 11S globulin (S. Damodaran, J. Agric, Food Chem., 36, 262 (1988)).
In the method for enzymatic decomposition up to now, however, it was not possible to specifically and exclusively decompose a specific component in soybean protein, possibly due to uncontrollable protein denaturation treatments, such as excessive heating, alcohol treatment etc. prior to proteolytic hydrolysis.
If it is possible to decompose exclusively specific components in soybean protein, a soybean protein having inherent functional characteristics could be obtained from a mixture of the respective components.